That's a great question. Even within the Bay Area cost will vary depending on what part of the area you are working in and what kind of finishing you will be doing.

For instance, a bedroom will likely be less costly than a kitchen because of the finish usually found in a kitchen.

Your best bet is to contact a few local builders when you have a good idea of what kind of addition you are interested in to get a ballpark figure.

I allow about $300/sf for remodels, $400/sf for additions and extras for kitchens and baths in the SF Bay Area. Most kitchens with appliances are $80K - $100K, regular bathrooms are $10K - $15K and master baths are $15K - $25K. You can beat these prices by going the Ikea /Home Depot sourcing route, but the State and County requirements are adding layer upon layer to the cost of building in California.

Depends on what your adding...materials used....etc...got example think of it in the terms of a car...are you adding a Chevy or Porsche?

Thank you all for answering. It is just an addition of 2 bedrooms and moving out two bedrooms to make them larger. I really am surprised at the price. At 400/sf I will probably hold off since $160,000 for adding 400 sq/ft is expensive.

Hi. I'm a local builder and am asked this question all the time. I'd offer the following two points in addition to the answers above. First, there is a survey done once a year by remodeling magazine and the national association of realtors where they collect data on hundreds of projects. Go to the following link and select the pacific region and San Francisco. http://www.remodeling.hw.net/remodeling-market-data/about-the-report.aspx.
Second, find a builder that you trust who does design+build. Give them your requirements and budget and ask them to design a project that can be built to your budget.